Microphone



Dec. 17, 1935. R T, RUTHERFORD I I2,024,705

MICROPHONE Filed Nov. 11, 1931 llllllllllzl'llvlllllll [lill/'111111111111111'.

l mz/ENTGR7 ROM/Lr 7T RUTHERFORD.

ATTORNEY Patented Dec. 17, 1935 UNITED STATES MICROPHONE Romilly T. Rutherford, Alameda, Calif., assignor to Donald K.`\Lippncott, Larkspur, Calif., as

trustee Application November 11, 1931, Serial No. 574,282

8 Claims. (Cl. 179-106) My invention relates to electro-acoustical transducers, or devices for eiecting a conversion between acoustic energy and electrical energy, and particularly to microphones for' converting sound impulses into corresponding electrical impulses.

Among the objects of my invention are: To provide a microphone capable of extraordinary delity of reproduction; to provide a microphone of utmost simplicity; to provide a microphone which is stable in operation and is not susceptible to changes in atmospheric pressure, temperature, or humidity; to provide a microphone which is rugged and may be handled carelessly without permanent damage; to provide a microphone which does not require mechanical precision in its construction; and to provide a microphone having the above enumerated characteristics which may be constructed cheaply and with` a minimum of equipment.

Other objects of my invention will be apparent or will be specically pointed out in the description forming a part of this specication, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a sectional view of a preferred embodiment of the microphone of my invention, connected to its accompanying amplifying vacuum tube.

Figure 2 is a transverse sectional view of the microphone shown in Figure 1, the plane of projection being on the line 2-2 in the first ligure.

Figure 3 is a sectional diagrammatic view of the active elements of the microphone shown in Figure 1, showing the circuit diagram of the connected amplifier.

Figure 4 is a similar sectional view of a modied form of microphone embodying my invention, and illustrating a slightly different amplifier arrangement.

Many electro-acoustical transducers have been constructed in the past which have been denominated electrostatic. This term has been justified in that the electrical forces acting are due to the electrostatic vector of an electric current, but they have not been truely electrostatic devices in that for their operation current has always been caused to flow in a complete electric circuit, even though such circuit did include a capacitative portion. The structure of such devices has been variously modied in accordance with the use to which the device was to be put, dimensions and materials being altered to produce the most advantageous conditions for conversion from sound into electric impulses or vice versa. In principle,

I however, these devices have comprised a. `condenser, one plate of which was vibratile. A biasing charge, usually provided by a high potential battery, was imposed between the two condenser 5 plates. For use as a microphone, a high resistance was connected in series between the two plates and the battery. When the vibratile plate was acted upon by a sound wave its distance from the other plate varied cyclically, producing cor- 1o responding cyclic variations in capacity. When the capacity increased current would ow from the battery into the two plates to maintain the potential between them at the same value. When the plates separated, 'a return ow took place due 15 to the decrease in capacity. This ow caused a voltage drop across the high resistance, and the vacuum tube amplifier was connected with its input circuit across this resistance, to amplify the voltage drop and produce the required sound cur- 20 rents.

When utilized to convert electrical pulsations into sound, the operation of these devices is merely reversed, voltage changes across the condenser plates producing changes in the attractive force 25 between them, causing vibration.

The transducer of my invention operates on a principle, which, although allied to that above described, is distinctly different in theory. When utilized as a microphone, for which the present 30 embodiment is best adapted, 'only two elements are required. These comprise a charged dielectric member and a conductive member mounted adjacent thereto, one of these members being vibratile sc that it's spacing from the other may 35 be changed by the action of a sound wave. The conductive member is connected by a single conductor to the control terminal' of a. potentialoperated amplifier, such as the grid of a vacuum tube. The conductive system, comprising the 40 metallic member, the grid, and the connected conductor, has a constant capacity, but as vibration occurs the distribution of charge on this conductive system is changed by its changing relations to the charge on the dielectric. Thus, if 45 the conductor is mounted adjacent the negatively charged surface, electrons are repelled from the member when its spacing from the dielectric is least, and forced onto the grid ofthe vacuum tube, increasing its negative charge. When the space between the conductive member and its charge increases the repulsion is less, and electrons return from the-grid to the conductive member, decreasing the negative charge on the grid.' There is an electronic flow along the single 55 conductor, even though there be no complete electric circuit andalthough the total capacity of the system does not change. The action is, of course, reversible, although owing to the extremely small capacity of the metallic member, considered as a free body, the impedance of the device is so great that the emciency of conversion from the output of an ordinary vacuum tube ampliner is small.

In practice, it is preferable to use a permanent electret in place of an ordinary frictionally charged dielectric. 'I'he electret comprises a dielectric material, usually a composition of resins and waxes of which probably the best discovered to date comprises a mixture of carnauba wax, beeswax, and rosin, in which has been melted and allowed to solidify in a strong electrostatic ileld. The opposing faces of the electret exhibit opposite electrostatic charges, no material deteriora-a tion in the strength of these charges having been observed over a period of seven years or more. Moisture and ionization of the air appear'to have no effect upon these charges, and neutralizing charges acquired, in the course of handling or otherwise, leak olf within a short time, restoring the effective charge upon the electret within a period of a few hours.

A preferred form of my invention is shown in Figure l. The electret I is formed with an active surface having a large number of closely spaced shallow depressions or recesses 2 in its front surface. The shape of these depressions is unimportant, except that they should be designed so that the ridges 3 between them account for as little of the entire surface of the electret as possible. For this reason, square or hexagonal depressions are to be preferred.

Any material or combination of materials which is capable of being electrized may be used in forming the electret. A satisfactory combination comprises carnauba wax one part, rosin one part, and beeswax one part. These proportions may'be varied widely, successful electrets having beenmade using pure carnauba wax, while mixtures of rosin and beeswax without the addition of carnauba have also been electrized. The composition named has proved desirable in that it does not soften below a temperature of about 80 centigrade, and is capable of taking extremely high electrizing charges.

The wax is cast in a. mold having a conduct= ing surface formed to act as a matrix for the depressions in its face. An electrode is suspended over the melted wax in the mold, and a high direct potential, in practice of the order of ten to fifteen thousand volts, is applied between the electrode and the conducting matrix. The wax is permitted to cool in the electrostatic eld thus applied, after which it is removed from the matrix and small holes 5 are drilled through the electret communicating with the recesses 2. A larger hole 6 is preferably also drilled through the wax to permit the passage of a conductor. 'I'his may conveniently be made in the center of the electret as shown, although its actual location is not important.

A metal diaphragm 1 is adhesively secured over the pitted surface of the electret. Various materials have been utilized in making diaphragms for this purpose. These materials have ranged from relativelyheavy lead foil (.003 inch) to aluminum leaf (.0001 inch). At present I prefer to use thin aluminum foil, preferably approximately .00025 inch thick. This combines the desirable degree of lightness and flexibility with reasonable permanence, but results have been obtained with all of the materials used.

A contact 9 leads from the diaphragm to the control electrode I0 of a head-amplifier tube II.

Both the microphone and the head-amplifier tube I I are preferably enclosed in a metal case I2, which includes a wire screen I3. covering the diaphragm, to'permit the passage of sound waves while shielding it from interferent iields, either electromagnetic or electrostatic. 10 The connections of the device are shown in Figure 3. Only the single conductor 9 connects the diaphragm and the amplifier tube. This tube may be a tetrode, as indicated in Figure 1, or a triode as shown in Figure 3. It is usually prefer- 15 able to bias the grid through a high resistance I5, by means of a battery I6, to prevent blocking of the tube. The tube output is of conventional form, the plate I1 being shown as connected through the primary of an audio frequency trans- 20 former I9 and plate battery 20 back to the iilament 2|. Resistance coupling may, however, be used in the output of the tube I I if desired.

The action of the device is as follows: When a sound wave reaches the diaphragm its com- 25 pression phase forces the foil into the 'recesses 2 to a slightly greater degree than normal, decreasing the mean spacing between the diaphragm and the electret. Assuming the front face of the electret to carry the negative charge, elec- 30 trons are repelled from the foil, flowing along the conductor 9 to the grid I0, thus increasing its negative potential and decreasing theplate current of the tube. When the rarefaction phase of the sound wave reaches the diaphragm the 35 latter is drawn out of the recesses, permitting a flow of electrons back to the diaphragm and decreasing the negative potential on the grid, thus permitting a greater plate current flow.

'I'he holes 5 serve to relieve pressure back of 40 the diaphragm, thus increasing its effective compliance and permitting greater motion. At the same time the flow of air through these holes with the vibration of the diaphragm increases damping and tends to'destroy any tendency toward 45 periodicity in the minute elementary diaphragms into which the foil is divided by the criss-cross supporting ridges 3.

It will be noted that the holes 5 are omitted in the showing of Figure 3. The device is en- 5o tirely operative when constructed in this manner, but its sensitivity is materially reduced.

A modified form of the device is shown in Figure 4. In this form the device comprises a conducting back-plate 25 having a recess 26 formed 55 in its front surface. stretched over the recess ls a fabric diaphragm 21, preferably of silk, which has been impregnated with wax and electrized. The diaphragm is secured to the back-plate by a clamp-ring 29. As in the case of the first form 60 a shielding container is preferably provided, this being omitted in the figure for the sake of simplicity. A conductor` 30 connects the back-plate 25 with the grid 3| of a head-amplifier tube 32. 'Ihe grid may be biased through a resistor as 65 shown in connectionwith the diagram of Figure 3, or it may be operated free. 'I'he same is, of course, true with the othery form of microphonel shown. It is desirable that conductivity between the grid and filament be reduced to as low a point as possible, but I have found that the biasing resistor is usually to be preferred since it increases the stability of operation of the device and lessens the liability of the tube blocking due to powerful signals.

When the electrets are first made, they retainl a powerful charge on their surface which masks the permanent electrization. These masking charges leak oi, so that in about twenty-four hours they are dissipated and the permanent charges, which are of opposite sign to the masking charges, become apparent on the opposite faces of the electret. Handling of the electrets also serves to form masking charges on the surface, so that the electrets appear temporarily to be discharged, As is the case when the electrets are rst formed, these masking charges also leak oi. Thus, when the microphone is first connected to the tube, its permanent charge has usually been quite thoroughly masked, and the sensitivity of the device is accordingly low. Within iifteen or twenty minutes, however, the masking charge has leaked off and the eective polarizing charge has therefore built up to a usable degree. Within three hours the charge closely approximates maximum.

It should be noted that the completeness of the masking by these surface charges depends to a large degree upon the amount of handling which the electret receives. It is rather difdcult to mask the permanent charge completely, and minor adjustments and connections may be made without materially reducing the sensitivity of the microphone. Since it is unnecessary, under ordinary circumstances, to handle the electret after it has once been set up, the time required for its full recovery after complete masking does not oier a material limitation to its use.

My invention, in its various forms, oiers many evident advantages. The fewn'ess and-` .nel

of its parts, and their low cost of manufacturV are obvious. It is unnecessary that the parts 'be accurately dimensioned, and there is hence no necessity for precision work in constructing any part of the device. In the ordinary condenser microphone, the most diilicult problem is insulating the closely spaced, accurately dimensioned parts against the relatively high biasing voltage which must be used. In the present device, since the biasing charge is carried entirely by insulating material, this problem does not enter. Furthermore, there is no danger of damage to the microphone from unduly loud sounds or from careless handling. The ordinary microphone, when hot must be handled with extreme care, since a contact of the diaphragm with the back-plate destroys the usefulness of the device. In the present case, should the diaphragm actually be forced into contactwith the bottom of one or more of the recesses, the only result is a temporary masking charge at the particular point contacted, and the total effectiveness of the device is not perceptibly changed.

It is of course possible to mount the microphone and its head-amplifier in separate vshield-y ing containers. In this case, the only precaution to be observed is that shielding surrounding the back of the electret does not actually contact with it. The more closely, the device may be made to approximate a truly unipolar arrangement the greater its effectiveness, and a grounded container in actual contact with the back of the electret, assuming the amplifier shield also toy be grounded, and connected to the control conpacity.

ductor through the biasing resistor, will apply a masking charge which `seriously impaired the eil'ectiveness of the device.

It will be obvious that many modifications of the device as shown may be made. The construc- 5 tion of the ordinary condenser microphone may be copied, the usual stretched diaphragm being used and the permanent electret substituted for the metallic back-plate. The constructions shown in Figures 3 and 4 may be'reversed, the recessed 10 surface being formed of metal and an electret diaphragm of impregnated fabric being used. Other modifications will suggest themselves to those skilled in the art. l

I claim:

l. The combination with a vacuum tube amplifier of a microphone comprising two closely spaced members, one of said members being metallic and the other being a permanent electret, and one of said members being vibratile, and a 2.0 single conductor connecting said microphone and said amplifier, said conductor being connected from said metallic member to the control electrode of said amplifier.

2. An electro-acoustic transducer comprising a 25 `support member having a recess formed in the surface thereof, and a diaphragm member mounted on said support and extending over said recess, one of said members being electrically conductive and the other being a permanent elec- 30 tret.

3. An electro-acoustic transducer comprising a permanent electret having a surface formed with a plurality of recesses therein, and a conductive i diaphragm secured directly to the portions of the 35- surface of said electret between said recesses.

4. An electro-acoustic transducer comprising a permanent electret having a surface formed with a plurality of recesses therein, and a diaphragm of metal foil secured directly to the portions of 40 the surface oi' said electret between said recesses.

5. An lelectro-acoustic transducer comprising a permanent electret having a surface formed with a plurality of recesses therein and having perforations through said electret communicating with 45 said recesses, and a conductive diaphragm covering vsaid recesses and in contact with said surface between said recesses. y

6. An electro-acoustic transducer comprising a metal back-plate formed with a recess in the 50 surface thereof, and a diaphragm of fibrous material impregnated with .electrized dielectric cov-V ering said recess.

7. The method of electro-,acoustic transduction which comprises the step of cyclically varying 55 the position of a conductive system with respect to an electrostatic charge, without changing the capacity of said system, to produce cyclic changes in the potential of said system.

8. 'I'he method of producing electrical varia- 60 tions corresponding to sound waves with a system comprising a dielectric and a conductor, which f comprises charging said dielectric electrostatically, and causing said sound waves to vary the means spacing between said dielectric and said ll conductor, whereby changes of potential are induced in said conductor without changes in ca- ROMILLY T. RUTHERFORD. 

